I've been using counter.social. They've had like 20,000 new subscribers in the last few days so it's very slow at the moment until things get upgraded. I've mostly been posting cat pics, so don't really know if there's much fandom related things. I'm not obsessed with a particular fandom recently so I haven't really looked, although I do see the occasional post. There's apparently groups as part of the paid features $5/month, but I haven't gotten around to upgrading to find out. I probably will at some point soon.